In a reusable pen having a two piece drive sleeve, the two halves of the drive sleeve are coupled during both dialing and dispense but are decoupled to enable the piston rod to reset when a new cartridge is fitted. A potential failure mode of such a design is that if the attached needle is blocked, or if the user forgets to attach a needle, and the user attempts to dispense a dose by pressing a dose button, then the dose button will begin to advance, the number sleeve will begin to rotate (counting downwards from the “set” dose) and the piston rod will begin to advance distally (in the direction of the cartridge), causing the deformable elements of the cartridge, for example a rubber bung and/or a rubber septum closing the needle end of the cartridge, to compress or deform until such time as the pressure from the cartridge bung against the piston rod becomes sufficient to resist the applied user force and the dispensing mechanism will then jam. This will typically occur within a few mm movement of the piston rod (i.e. within a few “units” of the number sleeve display). The increased resistance to movement of the dose button, and the jamming of the dispense mechanism, alerts the user to the fact that the mechanism is not working correctly (i.e. the medicament is not being dispensed), even though the number sleeve will now be displaying a number less than the “set” dose (i.e. the number sleeve appears to display a “partially dispensed” dose). If the user then releases pressure on the dose button, the deformable elements of the cartridge will tend to return to their original state, causing pressure on the piston rod which will cause it to move proximally (in the direction of the dose setting mechanism) which will in turn cause the device to go into a reset mode which will relieve the pressure in the system and will enable the dispensing mechanism to operate again, with the user force required to start the dose button moving again, having returned to the normal operating value, i.e. before the jam occurred. During reset the number sleeve, which is not affected by the resetting of the piston rod, will continue to display the “partially dispensed” dose. If the needle remains blocked, and, in an attempt to complete this undelivered or apparently partially delivered dose, the user repeats this cycle of pressing and releasing the dose button, whilst the device repeatedly jams and self-resets, the number sleeve will appear to be display that the device is dispensing medicament in stages of several units, with the number sleeve eventually returning to display zero units, i.e. the number sleeve appears to display a “completed dose”, when in fact no medicament is dispensed.
It is therefore an object of this invention to provide an improved and yet compact dose setting mechanism for a resettable pen device that removes this potential failure mode.
This is obtained by a dose setting mechanism as defined in claim 1. The dose setting mechanism of the present invention can be used in devices which use a two part drive sleeve that remains coupled during both dose dialing and dose dispensing and which only decouples when the user presses on the piston rod to reset the device e.g. replacing a cartridge. A dose setting mechanism according to the present invention comprises a dose setting member, e.g. a number sleeve, a dose button, a drive member, e.g. a drive sleeve, for driving a piston rod in a distal direction during dose dispensing, a first clutch and spring means.
Typically, the drive member comprises a proximal drive member and a distal drive member which are releasably coupled via the first clutch. The spring means are designed for biasing the proximal drive member and the distal drive member in the coupled state such that during dose setting and dose dispensing relative rotation of the two parts of the drive member is not allowed. Preferably, the first clutch rotationally couples the proximal drive member and the distal drive member during dose setting and dose dispensing and rotationally decouples the proximal drive member and the distal drive member during resetting of the mechanism. The operation of the dose button by the user, e.g. during dispense, increases the compression of the spring means, and therefore prevents decoupling of the clutch during dispense when a force is applied to the piston rod.
Further, engaging means are provided which are associated to the proximal drive member and to the dose setting member, respectively. Said engaging means are designed and arranged such that the proximal drive member entrains the dose setting member in the distal direction during dose dispensing but allows a relative axial movement of the proximal drive member with respect to the dose setting member in the proximal direction at all other times. If the mechanism is a resettable mechanism it is preferred that the engaging means, if applicable together with further means, allow a relative axial movement of the proximal drive member with respect to the dose setting member in the proximal direction only when a dose button for transmitting a dispensing force applied by a user to the mechanism is pressed during or following dose dispense, and resists relative axial movement when the dose button is not depressed during reset. In normal, non-dispensing, operation the proximal drive member and the dose setting member will be biased into contact at the engaging means by the force applied by the spring means. According to a preferred embodiment of the invention, the engaging means may comprise hooks provided on fingers protruding from the proximal end of the proximal drive member and a corresponding inwardly protruding flange on the proximal end of the dose setting member.
The above mentioned design of the dose setting mechanism allows the proximal half of the drive sleeve to move proximally with the distal half of the drive sleeve in the event that the dose button is released with residual bung compression, e.g. during the potential failure mode of the user attempting to dispense medicament with a blocked needle. If both halves move together axially then they do not decouple and the device does not enter the reset mode thus avoiding the potential failure mode. In this case if a new, unblocked, needle is fitted, then the spring means, which will have been further compressed by the proximal movement of both halves of the drive member, will cause the whole drive member to be driven distally again, dispensing medicament from the needle and relieving the compression in the cartridge, and restoring the correlation between the displayed dose on the number sleeve and the set dose in the mechanism.
Typically, the engaging means comprise a flange or protrusion provided on the proximal drive member and a corresponding flange or protrusion provided on the dose setting member. Thus, the engaging means are integrally formed on the proximal drive member and the dose setting member, respectively. As an alternative, the engaging means may comprise separate components which are attached to the proximal drive member and/or to the dose setting member.